You seem confused with the word "need". Say I need a card that will performs flawlessly at resolution of 800x600, do I need to buy a 4890? Absolutely no. You may want to but you don't need it yet (unless you upgrade your monitor)
Resolution in gaming is a form of specification of what he needs. That's why resolution is always mentioned and considered in reviews including level of detail, AA/AF, etc. It all consists of what the user needs for his system, not what he wants to have for the system.
A gts 250 is enough at that settings. Even with AA/AF, it would surely do. You could also go the ATI way with their 4850. Waiting for dx11 cards is a matter of personal preference. IMO, the only reason for me to wait for them is because dx10 card's prices might drop even further. It depends on what you want right now.